1. Field of the Invention
The present invention is directed to pulsed lasers, and more particularly to pulsed lasers for producing high power mode locked light pulses.
2. Description of the Related Art
A laser, in its simplest configuration, operates in a “continuous wave” (cw) mode, in which the power output is relatively constant over time. For many applications, such as cutting, drilling, and investigation of media properties, it would be beneficial to use a pulsed laser, in which the power output is extremely high over a very short period of time. In such a pulsed laser, the output power is effectively “stored up” in the laser cavity over a period of time, then released in a short burst. During the duration of the laser pulse, known as the pulse width or pulse duration, the output power of the laser may be many orders of magnitude larger than the continuous wave output power. A preferable method of generating these short pulses is known as mode locking.
A physical description of mode locking may be found in A. E. Siegman, “Lasers” (University Science Books, 1986), pp. 1041–1061. The output from a mode locked laser is a typically a train of regularly spaced pulses of temporal separation T, where T equals the round-trip time for light inside the laser cavity, although the laser cavity may be modified to produce a single pulse with a manual trigger. Mode locked lasers can routinely produce pulses with durations of picoseconds.
There are essentially three categories of mode locking, known as “active”, “passive” and “active-passive”. Each of these is described in greater detail.
In active mode locking, a modulator is placed inside the laser cavity, such as an acousto-optic or electro-optic modulator. The modulation frequency should be well tuned to the round-trip frequency inside the cavity. The laser begins to oscillate in the form of a short pulse that circulates around inside the laser cavity, passing through the modulator on each round trip just at the instant when the modulator transmission is at its maximum. On each pass through the modulator, the pulse becomes shorter, because the leading and trailing edges of the pulse are attenuated more than the peak of the pulse. Because the modulation is controlled by an external device, the pulse emission from an active mode locked laser is inherently stable (i.e., the pulse energy is fairly constant from pulse to pulse, and the pulse emission is regular and predictable over time) and is, therefore, easily synchronized with other equipment. One drawback to active mode locking is that the pulse durations are relatively long, on the order of tens of picoseconds.
Various prior art active mode locking techniques are disclosed in U.S. Pat. No. 3,586,997, issued to Kinsel, U.S. Pat. No. 4,665,524, issued to Cotter, and U.S. Pat. No. 5,014,277, issued to Van Driel et al.
In passive mode locking, an optically nonlinear material is placed inside the laser cavity. The nonlinear material is generally absorbing at low optical intensities, but then saturates and becomes more transparent (less absorbing) at high intensities. Because of this behavior, the nonlinear material is usually referred to as a saturable absorber. An optical pulse, which usually begins its life as a statistically indeterminate noise spike, grows in intensity as it travels through the laser cavity, due to pumping of the gain medium. Once the peak intensity of the noise spike reaches the saturation level of the nonlinear material, the peak of the noise spike receives less attenuation than the leading and trailing edges, and the duration of the noise spike decreases upon each subsequent pass through the nonlinear material, until it reaches a suitably short length and becomes the output pulse. Two primary advantages of passive mode locking over active mode locking are: (1) Passive is simpler to implement, because it doesn't require an external modulator exactly tuned to the cavity round-trip time, and (2) the pulses produced by passive mode locking are significantly shorter than for active mode locking. Two disadvantages of passive mode locking are: (1) The pulse output is less stable than for active mode locking (i.e., the power varies from pulse to pulse, and the time between pulses varies statistically), and (2) a passive mode locked laser cannot be synchronized with other equipment.
Various prior art passive mode locking techniques are disclosed in U.S. Pat. No. 3,978,429, issued to Ippen et al., and U.S. Pat. No. 4,435,809, issued to Tsang et al.
In active-passive mode locking, both active and passive mode locking methods are employed simultaneously. A laser cavity may contain both a modulator and a saturable absorber. Combining both active and passive techniques in the same laser provides the advantages of both short pulse duration and pulse stability (i.e., the pulse energy is fairly constant from pulse to pulse, and the pulse emission is regular and predictable over time). In addition, active-passive mode locked lasers are also easily synchronized with other equipment.
A prior art active-passive mode locking technique is disclosed in U.S. Pat. No. 4,019,156, issued to Fountain et al.
The most common saturable absorber material used by passive (or active-passive) mode locked lasers is a liquid dye, which is continuously recirculated in a flat stream that crosses the beam inside the laser cavity. The dyes perform well optically, with desirable characteristics such as good absorption saturation contrast and fast recovery times. Unfortunately, they break down chemically after a certain amount of use, and must be replaced periodically, often at inconvenience and expense to the user. The dyes are often toxic, requiring careful handling by the user, or a service call to a maintenance technician. Because of the dye's inherent chemical instability, and the inconvenience of periodic replacement of the dye, a great deal of effort has been spent on finding solid-state saturable absorbers to replace dyes in mode locked lasers.
A semiconductor material can be used as a saturable absorber, as disclosed in U.S. Pat. No. 6,466,604, issued to Kopf. In this prior art, the semiconductor material is grown as a thin film structure on an optical component, and proves very convenient to use. However, the semiconductor material shows poor optical qualities, specifically a low contrast ratio and a low damage threshold. Additionally, the material suffers damage when used in high power lasers.
Recently, a new category of material has appeared, in which tiny semiconductor crystals, sometimes referred to as microcrystals, are grown in an amorphous glass matrix. The microcrystals are so small that they show quantum confinement effects in three dimensions, and are given the name quantum dots. Quantum dots are extremely useful in the fabrication of nonlinear optical devices, and an example of PbS used as a quantum dot material is disclosed in U.S. Pat. No. 5,449,645, issued to Borrelli et al.
Quantum dots have been used as saturable absorbers for a variety of quantum dot materials at a variety of wavelengths. See, for instance, V. G. Savitski, et al “PbS-doped phosphate glasses saturable absorbers for 1.3-μm neodymium lasers”, Applied Physics B, vol. 75, No. 8, pp. 841–846, 2002; K. Wundke et al “PbS quantum-dot-doped glasses for ultrashort-pulse generation”, Applied Physics Letters, vol. 76, No. 1, pp. 10–12, 2000; A. M. Malyarevich et al, “Nonlinear optical properties of solgel-derived glasses doped with copper selenide nanoparticles”, J. Opt. Soc. Am. B, vol. 17, No. 4, pp. 572–578, 2000; G. Tamulaitis et al, “Optical nonlinearities of glass doped with PbS nanocrystals”, J. Applied Physics, Vol. 88, No. 1, pp. 178–182, 2000.
One drawback to quantum-dot doped glass saturable absorbers is that they tend to have relatively poor thermal properties. For relatively low powers, they perform adequately. But when used for high power pulse generation, the repetition rate is typically kept low, in order to prevent thermal damage to the saturable absorber.
There is a need for a mode locked laser that does not suffer the disadvantages of conventional mode locked lasers having a dye solution as the saturable absorber, while nonetheless being capable of high output power, short pulse duration, and fast repetition rate.